FIELD OF THE INVENTION
[0001] This invention relates to a direct injection diesel engine provided with an exhaust
gas recirculation system, and more particularly to a method of reducing emission of
smoke when exhaust gas is recycled.
BACKGROUND OF THE INVENTION
[0002] Exhaust gas recirculation systems (referred to hereafter as EGR systems), wherein
part of the exhaust gas from an engine is recycled in the air intake to reduce peak
temperature and pressure in the combustion chamber, are a recognized means of reducing
emission of NOx in the exhaust from direct injection diesel engines.
[0003] Although such EGR systems do reduce the amount of NOx generated, they also have the
disadvantage that generation of smoke (particulate matter) tends to increase, since
the recirculation of exhaust gas effectively reduces the oxygen concentration in the
combustion chamber.
[0004] The fuel injection valves of diesel engines are generally fitted to the cylinder
head at an oblique angle to avoid interference with air intake or exhaust valves.
This however leads to uneven fuel injection inside the chamber so that air is used
less efficiently where there is a high concentration of fuel, and more smoke is generated
when exhaust gas is recycled.
[0005] A two stage fuel injection valve for diesel engines is disclosed in, for example,
Jikkaisho No. 61-160271 and No. 61
-162572 published by the Japanese Patent Office.
[0006] In this valve, a small amount of fuel is injected in an initial lift, and after this
initial amount of fuel has ignited, a larger amount of fuel is injected in a second
lift. This system was intended to reduce the ignition delay in the main fuel injection,
help the the combustion flame to spread smoothly and rapidly throughout the chamber,
reduce combustion noise and emission of NOx, and generate less smoke.
[0007] In this type of fuel injection valve however, if the needle valve is even slightly
decentered in the initial injection when the injection pressure is low, the injection
of fuel into the chamber is liable to be uneven, conversion of fuel to fine droplets
is difficult where fuel is in high concentration, and as air is not used efficiently
in the combustion, even more smoke may be generated.
[0008] Thus, even if an EGR system is combined with a two stage fuel injection valve and
emission of NOx is decreased, smoke generation often increases. It was therefore difficult
to reduce both NOx and smoke simultaneously.
SUMMARY OF THE INVENTION
[0009] It is an object of this invention to promote uniform mixing of fuel and air in the
combustion chamber of direct injection diesel engines, and thereby to reduce generation
of smoke when exhaust gas is recycled.
[0010] To achieve this object, this invention provides a diesel engine with a combustion
chamber formed by a cylinder and piston where fuel is injected into the chamber directly,
the engine comprising means for recycling part of the exhaust gas into the air intake
depending on the running condition of the engine, a fuel injection valve fitted to
the cylinder head of the cylinder substantially parallel to the cylinder axis and
facing the center of the chamber which lifts in two stages depending on the fuel pressure,
and two air intake valves and two exhaust valves disposed around the fuel injection
valve.
[0011] The fuel injection valve is provided with a needle valve which is subjected in stages
to the force of two return springs with different spring loads, fuel being injected
from an injection nozzle when the needle valve lifts. The needle valve is also provided
with a guide which slides on the inner circumference of the injection nozzle and which
guides the needle valve such that it is concentrically disposed with respect to the
injection nozzle.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Fig. 1 is a schematic diagram of one preferred embodiment of this invention.
[0013] Fig. 2 is a control map of the amount of exhaust gas recycled which is set on the
EGR system in the same embodiment.
[0014] Fig. 3 shows a vertical section through the cylinder head and upper part of the cylinder
in the engine of the same embodiment.
[0015] Fig. 4 shows a plan of the cylinder head as viewed from the cylinder in the same
embodiment.
[0016] Fig. 5 shows a vertical section of the fuel injection valve in the same embodiment.
[0017] Fig. 6 is a graph showing the relation between amount of fuel injected and oxygen
concentration in the combustion chamber.
[0018] Fig. 7 is a graph showing the relation between amount of fuel injected and amount
of smoke generated.
[0019] Fig. 8 is a graph showing the relation between amount of NOx and amount of particulate
matter generated.
[0020] Fig. 9 is an enlarged view of the fuel injection nozzle in Fig. 5.
[0021] Fig. 10 is a cross sectional view of the fuel injection nozzle taken along the line
A-A of Fig. 9.
[0022] Fig. 11 is a front elevation of the fuel injection nozzle.
[0023] Fig. 12 is an enlarged view of a part of Fig. 9.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] Referring to Fig. 1, a direct injection diesel engine 1 has an air intake manifold
52, an exhaust manifold 53 and a fuel injection pump 54.
[0025] Air intake manifold 52 and exhaust manifold 53 are connected each other via exhaust
gas recirculation passages (hereafter referred to as EGR passages) 55 and 56, and
an exhaust gas recirculation valve (referred to hereafter as an EGR valve) 57 is provided
between these EGR passages 55 and 56. Opening and closing of the EGR valve 57 is controlled
by a diaphragm 90 acting as a negative pressure actuator.
[0026] A throttle valve 59 which is driven by a diaphragm 60 acting as a negative pressure
actuator, is also provided upstream of the confluence of the air intake manifold 52
and EGR passage 56. When the EGR valve 57 opens, exhaust gas is recycled, and the
amount of gas recycled is increased by reducing the aperture of the throttle valve
59.
[0027] The diaphragm 90 acts under a negative pressure transmitted from a vacuum pump 58
driven by the engine 1 via a negative pressure passage 61, negative pressure control
valve 62 and negative pressure passage 65.
[0028] The diaphragm 60 acts under a negative pressure from the negative pressure passage
61 via a negative pressure control valve 63 and negative pressure passage 64.
[0029] The pressure control valves 62 and 63 are connected by signal circuits to an electronic
control unit 68, arid control the negative pressures delivered to the diaphragms 90
and 60 respectively according to output signals from this control unit. The fuel injection
pump 54 is equipped with a lever aperture sensor 66, which detects the aperture of
the pump lever that controls the amount of fuel injected (engine load) concurrently
with an accelerator pedal not shown, and an engine speed sensor 67 that detects engine
speed. These sensors are both connected to the control unit 68 via signal circuits.
[0030] The control unit 68 is provided with the EGR control map shown in Fig. 2. Based on
input signals from the sensors 66 and 67, and on this EGR control map, the control
unit 68 computes pressure control values and outputs them to the valves 62 and 63.
[0031] Figs. 3 and 4 show the construction of the cylinder head 10 and upper part of the
cylinder 2 in this diesel engine. A piston 3 is fitted such that it is free to slide
in the cylinder 2, and a cavity 4 on the top of piston 3 forms a combustion chamber
5 in the cylinder 2.
[0032] A fuel injection valve 6 is provided in the center of cylinder head 10. This fuel
injection valve 6 is disposed parallel to the center axis of the cylinder 2, and a
fuel injection nozzle 7 at its tip is situated close to the center of the combustion
chamber 5.
[0033] Two air intake valves 8 and two exhaust valves 9 are also disposed around the nozzle
7 in combustion chamber 5. These valves 8 and 9 are arranged parallel to the center
axis of the cylinder 2.
[0034] Air intake ports 21, 22 connected to the valves 8, and exhaust ports 23, 24 connected
to the valves 9, are also provided in the cylinder head 10.
[0035] The valves 8 and 9 are driven in opposition to valve springs 15 and 16 by rocker
arms 13 and 14 supported such that they are free to oscillate in the cylinder head
10. Air intake cams 11 and exhaust cams 12 which rotate synchronously with the revolution
of the engine are also provided in the upper part of the cylinder head 10. These cams
11 and 12 open and close the valves 8 and 9 with a specified timing in synchronization
with the engine by pressing the rocker arms 13 and 14 down against the force of the
valve springs 15 and 16.
[0036] Fig. 5 shows the structure of the fuel injection valve 6. This valve 6 is provided
with an injection nozzle 7, and a needle valve 34 seated in a seat 41 formed at the
tip of the nozzle 7, as shown in Figs. 9 and 12. A plurality of spray holes 47 as
shown in Fig. 11 ire formed in the seat 41 facing combustion chamber 5. These spray
holes 47 open when the needle valve 34 lifts away from the seat 41, and spray fuel
supplied from a fuel inlet 31 into the combustion chamber 5.
[0037] The spray holes 47, which are of equal surface area, are arranged radially in seat
41, spaced at equal intervals apart, and inclined at equal angles to the nozzle axis.
[0038] The injection nozzle 7 is arranged parallel to the axis of cylinder 2 in the center
of the combustion chamber 5, and is provided with the above-mentioned spray holes
47. Fuel injected from near the center of the combustion chamber 5 is thus dispersed
evenly throughout the chamber, its concentration is distributed evenly, and it mixes
with air more effectively. Air is therefore utilized more efficiently in the combustion.
[0039] The needle valve 34 is supported by a first return spring 42 via a shim 37 housed
in a nozzle holder 38 which holds the nozzle 7, and via a push rod 39. This needle
valve 34 is provided with an outer skirt 49 of large diameter adjacent to a small
chamber into which fuel is led from the inlet 31, and it moves upwards in an axial
direction depending on the fuel pressure acting on the skirt 49. 36 is a stopper which
adjusts the full lift of the valve 34, and 44 is a set screw which sets the spring
load of the first return spring 42.
[0040] A spring seat 48 supported by a second return spring 40 is provided above the shim
37. The first return spring 42 and second return spring 40 have different spring loads.
[0041] The needle valve 34 is first lifted by the fuel pressure acting upon the skirt 49
until the shim 37 touches the spring seat 48 while compressing the first return spring
42, and an initial fuel injection takes place through the spray holes 47 for the duration
of this initial lift L₁.
[0042] When the fuel pressure rises so that it exceeds the set load of the second return
spring 40, the needle valve 34 begins to lift again while compressing the second return
spring 40 via spring seat 48 in addition to the first return spring 42. The needle
valve 34 continues to lift until it touches the stopper 36, and fuel is injected at
high pressure through the spray holes 47 for the duration of this full lift L₂. The
needle valve 34 therefore lifts in two stages. Initially, a small amount of fuel is
injected, and a larger amount is then injected in a main injection. The effect of
this is that, once the initial amount of fuel has ignited, the larger amount injected
in the main injection can be burnt smoothly.
[0043] A guide 43 which slides on the inner circumference of the injection nozzle 7 is provided
on the needle valve 34. As shown in Fig. 10, this guide 43 has a passage which allows
fuel to flow vertically up or down. Due to this guide 43, the valve 34 moves concentrically
with respect to the injection nozzle 7, the cross section of the annular flow path
produced between the needle valve 34 and seat 41 due to the lift of the valve is maintained
uniform, and fuel is injected into the combustion chamber 5 more uniformly.
[0044] As shown in Fig.s 6 and 7, when exhaust gas is recycled, the O₂ concentration in
the combustion chamber 5 falls and emission of smoke tends to increase. According
to the present invention, however, the guide 43 renders the fuel injection uniform,
and air is utilized more efficiently especially in the initial injection. As shown
in Fig. 7 therefore, the smoke level when exhaust gas is recirculated is far lower
than in prior art which are not provided with the guide 43.
[0045] As shown in Fig. 2, the operating region in which EGR can be performed is thus enlarged,
and a large amount of exhaust gas can be recycled even on partial load. T'he result,
as shown in Fig. 8, is that both particulate matter and NOx are reduced.
[0046] By providing a guide 43 on the needle valve 34, the outer diameter of the injection
nozzle 7 becomes fairly large. In engines which have one air intake valve and one
exhaust valve per combustion chamber, therefore, the diameters of these valves can
not be made large, to prevent them interfering with the injection nozzle, and it is
difficult to maintain the requisite air intake and exhaust capacity.
[0047] According to the present invention, however, two air intake valves 8 and two exhaust
valves 9 of small diameter are provided around the injection nozzle 7. It is therefore
possible to retain sufficient installation space for the injection nozzle 7 and sufficient
surface area for the valves 8 and 9, i.e., to provide sufficient air intake and exhaust
capacity without interfering with the nozzle 7.
[0048] Further, in the initial lift of the injection valve 6, the gap between the tip of
the needle valve 34 and the seat 41 is only of the order of 10 - 40 µm, and there
is therefore a tendency to stick when the temperature rises. Cooling passages can
however be provided between the air intake and exhaust valves 8 and 9 of the cylinder
head 10 and on the outer surface of the injection nozzle 7 so that the cooling performance
of the nozzle is increased, and the operating performance of the needle valve 34 is
maintained.
[0049] The foregoing description of a preferred embodiment for the purpose of illustrating
this invention is not to be considered as limiting or restricting the invention, since
many modifications may be made by those skilled in the art without departing from
the scope of the invention.
1. A diesel engine having a combustion chamber formed by a cylinder and a piston into
which high pressure fuel from a fuel injection pump is injected directly, comprising:
means for recirculating part of the exhaust gas into the air intake depending on
the running condition of the engine,
a fuel injection valve supported in the cylinder head of said cylinder substantially
parallel to the cylinder axis and facing the center of said combustion chamber wherein,
by lifting a needle valve in two stages according to the fuel pressure, said high
pressure fuel is injected in stages into the combustion chamber from the injection
nozzle,
a guide on said needle valve which slides on the inner circumference of said injection
nozzle and guides said needle valve concentrically, and
two air intake valves and two exhaust valves disposed symmetrically around the
fuel injection nozzle.
2. A diesel engine as defined in Claim 1 wherein said exhaust gas recirculating means
comprises an exhaust gas recirculating passage connecting an air intake passage and
an exhaust gas passage, an exhaust gas recirculation control valve inserted in this
exhaust gas recirculating passage, an air intake throttle valve situated upstream
of the confluence of said air intake passage and exhaust gas recirculating passage,
and a control unit which opens and closes these valves according to the running condition
of the engine.
3. A diesel engine as defined in Claim 2 wherein said exhaust gas recirculation control
valve and said air intake throttle valve are driven by negative pressure actuators.
4. A diesel engine as defined in Claim 3 wherein said negative pressure actuators are
driven by a negative pressure adjusted by a negative pressure valve controlled by
a control unit, and wherein said exhaust gas recirculation control valve is opened
and said air intake valve is throttled at least when the engine is running on partial
load.
5. A diesel engine as defined in Claim 1 wherein said air intake valves and exhaust valves
are fitted parallel to the cylinder axis, and these air intake and exhaust valves
are each connected to two air intake ports and two exhaust ports respectively.
6. A diesel engine as defined in Claim 1 wherein said fuel injection valve is provided
with a needle valve situated inside the injection nozzle, a first spring which acts
on this needle valve in such a direction as to close it, a spring seat which adjusts
the initial lift of said needle valve, and a second spring which acts on this spring
seat in such a direction as to close said needle valve.
7. A diesel engine as defined in Claim 6 wherein the seat supporting said needle valve
forced in said injection nozzle, is provided with a plurality of spray holes of equal
area which are disposed radially around the nozzle axis, spaced at equal intervals
apart and inclined at equal angles to the nozzle axis.